Coating process for polyethylene and composite articles thereby obtained



COATING PROCESS FOR POLYETHYLENE AND COMPOSITE ARTICLES TIEREBY OBTAINED Hamid G. Hahn and wilhelm E. wanes, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application May 25, 1956, Serial No. 587,361

15 Claims. (Cl. 117-72) This invention relates to a process for coating polyethylene films and other shaped articles with resinous layers comprised of a tripolymer of vinylidene chloride, acrylonitrile and a functionally basic ethenoid monomer. It also has reference to the coated articles, particularly films, which may thereby be obtained.

Films and other shaped and molded articles of polyid States arent" ethylene have many attractive characteristics including,

in general, good physical properties and, in particular, excellent behavior an-d resistance to embrittlement at low temperatures. Despite these attributes, however, shaped articles of polyethylene frequently may not have a satisfactory ability to provide effective barriers against the transmission or permeation of certain gases and vapors, particularly oxygen. Furthermore they do not always have suitable clarity and transparency and are prone to become scuffed and marred on their surface despite their otherwise strong and tough characteristics.

While certain of these shortcomings of polyethylene might be compensated for by coating it with various resinous materials having complemental properties to provide compositefarticles having more completelyysatisfactory characteristics, difficulties have been encountered in the derivation of such articles. `This is usually because. of the inefficient bonding and poor a-dherance that is experienced when most resinous materials are attempted to be coated on polyethylene surfaces in order to form composite structures. Polyethylene film and other articles, as is well known, commonly have a `smooth and sleek, relatively slippery and wax like surface which is poorly adapted to provide for suitable adhesion or anchorage of applied materials by mere physical attachment. Furthermore, the relatively inert chemical nature of polyethylene resists the efficient attachment of most materials by chemical inter-linkage or bonding.

It is among the principal objects of the present invention to provide an efficient and eective method for coating polyethylene films and other articles with tightly adhering integral layers of a resinous tripolymer of vinylidene chloride, acrylonitrile and a functionally basic ethenoid monomer (which is .polymerizable while retaining its pendant, basic functional groups available for further combination) so as to provide useful composite structures of polyethylene films and articles and the resinous coatings on the articles which are particularly adapted for employment at relatively low temperatures as fiexible materials for packaging and the like that have improved gas and vapor barrier characteristics, better clarity and a more senti-resistant surface.

According to the invention, polyethylene articles may be effectively provided with a tightly-adhering and firmly anchored coating of a resinous tripolymer of vinylidene chloride, acrylonitrile and a functionally basic ethenoid monomer by a method which lcomprises sulfonating the surface of Vthe polyethylene article and subsequently applying a layer of the functionally basic resinous tripolymer v of vinylidene chloride, acrylonitrile and a functionally 2,786,783 Patented Mar- .25

basic ethenoid monomer over the sulfonated polyethylene surface.

The composite articles (as illustrated by the composite film which is depicted perspectively in Figure 1 and cross'- sectionally in Figure 2 and by the coated tubular article shown perspectively in Figure 3 of the accompanying drawing), which advantageously may be obtained by practice of the invention, possess the desirable and beneficial characteristics of polyethylene while having improved gas and vapor barrier properties, especially to oxygen, greater resistance to scufting and better clarity. Composite film structures in accordance with the invention, for example, may generally be obtained with only a very slight degree of haziness which is not discernible upon ordinary visual inspection and which has been reduced almost to the vanishing point from the haze that is present in the original polyethylene film. ln addition, the applied coating, in most instances, is. extremely difcult if not completely impossible to strip from the surface of the sulfonated polyethylene film or other article by ordinary physical methods. 1

Composite film structures may advantageously be prepared in accordance with the present invention. Such film structures have particular utility as wrapping and packaging materials for foodstuffs and other articles which are intended to be handled or maintained and stored at relatively low temperatures in a refrigerated or frozen condition. Other composite structures including tubes, hose, flexible bottles and other containers such as cartons and boxes may also be made with advantage by practice of the invention.

Advantageously, relatively thin polyethylene films are employed in order to obtain composite film structures according to the invention. Beneficial results, for example, are readily obtainable with films having a thickness which is not greatly in excess [of about 20vmils. lt is frequently even more beneficial to employ polyethylene films having a thickness between about 0.5 and 5y mils. The polyethylene film which is employed in the practice of the present invention as well as other polyethylene articles may be derived from polyethylene of any nature. The polymers of ethylene which are employed may, for example, be similar to those which sometimes are referred to as polythenes and which may be obtained by` polymerizing ethylene in a basic aqueous medium and in the presence of polymerization-favoring quantities of oxygen under relatively high pressures in excess of 500 or 1,000 atmospheres at temperatures which may be between 150 and 275 C. Or, if desired, they may be similar to the essentially linear and unbranched polymers which have been referred to as ultrathenes and which ordinarily have greater apparent molecular Weights (as may be determined from such characteristics as their melt viscosities and the like) in excess of about 40,000;` densities of about 0.94-096 gram per cubic centimeter; and vmelting points in the neighborhood of 12S-135 C. They are ordinarily found to have a more crystalline nature than conventional polyethylenes and may contain less than 3.0 and even less than 0.3 methyl radicals per methylene groups in the polymer molecule. The essentially linear and unbranched polymers of ethylene may be obtained under relatively low pressures o f 1 to 100 atmospheres using s uch catalysts for polymerizing the ethylene as mixtures of' strong reducing agents and compounds of Group lected from the group consistingvof concentrated sulfurie acid containingj at leastabout 98 percent by weight of H2804, oleum, anhydroussolutions of oleum and free sulfur trioxide. Advantageously, the sulfonated surface may be washed free from excess sulfonatiii'g efagent'prior to the application of the resinous coating.

,Whilerhe surfaceV sulfonation ofthe polyethylene ,may be accomplished with any of the mentioned sulfonating reagents,.it.i s usually more advantageous to employ oleurn (Which is sometimes known as fuming sulfuric acid) containing .from trace amounts to about l percent `by weight of dissolved, free sulfur trioxide.` If desired,r the oleurn can also be employed benecially in anhydrous solutions with other materials such as acetic lanhydride and the like which permit effective quantities of the sulfur trioxide to be available in an amount whichy is equivalent to thatwhich is providedvin the oleum. ln certain instances, it may be convenient to employ free sulfur trioxide vapors which, beneficially, may be diluted to a concentration as low as percent or less, for example, with. a suitable inert gas `such, as nitrogen.

, Generally, a Ysatisfactory degree, of surface sulfonation maybe obtained when operating at temperatures between the freezing point `of the sulfonating reagent and about 150 C. for periods of time ranging from matters o fn 1e,rely severalV seconds or even almost instantaneous periods which involve mere fractions of seconds tohours. Frequently, when an oleum is employed which contains dissolved, free sulfur trioxide in amounts ranging from traces to about 10 percent by weight, the surface sulfonation may be performed suitably at an operating temperature of about 50 C. within a time period of about ve minutes. t Y

The `degree of surface sulfonation which is obtained on the polyethylene article predetermines to a great extent they bonding that may be obtained between the polyethylene and the subsequently applied resinous coating. For `all practical purposes, it precursesthe results which may bevrealized by practice of theirivention.A t The degree of sulfonation that-may beg obtained in any particular iii- `stance is ysomewhat interdependent on thewnature of the .polyethylene andthe specicwphysicalforrn or structure of4` the polyethylene article-*that is, being treated, the strength or effective -sulfurvtrioxidei concentration of the reagent, .the Operatingtemperature andthe-lies@ 0f. the treatment. Carewshould be taken-[toavoidsulfonation conditions which may be tootdrasti cas may occur when Yan Qleum'is employed with a relatively hightfree sulfur .triogride contentat too high a =temp'erature. or for too long a period ettim- .Care shwldls0-bs-takn foremploymore moderate treatingconditionsuupon' more deli- .Cate Strllf-iulesisuh as ;exspts ,nally.fns films and the like- If such. precautiens, .are .not assduouslv A Q bserved. the PolyethyleneA article mar be Caused. t0 .decompose and degrade resulting in its being darkened and discolored to an intolerable extent. Conversely, optimum coating results may not be obtained if overly weal; sulfonating conditions are employedk which may not sulfi- Venfly mcdfy the Surface Qf the ,Polyethyleneertisle 't0 permit Va suitably beneficial quantity of the coating to be effectively and permanently applied thereto. v ,Y

The resinous tripolymer that may be employed for f, coating the surface of the ,sulfonated polyethylene may contain between about l0` and k9,0 percent by weight of Vvinylidene chloride, between 5 and 60 percent by weight vofacrylonitrile, and between l and percent by weight of the functionally basic ethenoid monomer` polymerized 4inmthe tripolymer molecule..v Advantageously, f it Vmay contain between about 5 0. and 70 percent by weight ffth'e vinyliden'e chloride, between y30V and 50 Apercent by weight of the acrylonitrile and between y6 and l2 percent by vweight of the functionally basic ethenoi'd monomer. The functionally basic ethenoidV monomer that is employed,

Vvas is apparent, is` a material that retains a functional ,basicity `and imparts. thistcharacteristic tothe resinous ttripyridine and the like is employed for this purpose. If desired, however; such"functiona`1ly basicl ethenoid mon; omers as N-dimethylamino ethyl acrylate, 2-rnorpholino ethyl acrylate and the like may also be employed with benefit. The functionally basic resinous tripolymer may advantageously be prepared by aV suspension polymerization technique using organic peroxy catalysts in accordance with the disclosureycontained in the copending application of Marion H. RectorY andHaro'ld G. Hahn for a Process for` Producing Haloethylenic Polymers Having Enhanced Dye Rec'ep tivity,.having Serial No; 57i-,888 which was tiled on March 16, 1956, inv/bleh the polymerization is conducted in the0 presence of an acid that is capable of neutralizing .theA basicity of the pendant basic groups of the functionally basic ethenoid monomers during the polymerization to avoid degradation of the polymer product.

The functionally basicvresinous tripolymer may be utilized suitably in several Ways in order to obtain a coated layer of the resin on the vsuliionated polyethylene film or other article. They may be cast on the sulfonated polyethylene surface froma dissolved composition formulated with suitable solvents, such as in lacquer formulations or they may be deposited with polymerizing mixtures of the monomeric substances which are polymerized in they desiredresin. The casting may be accomplished by various spread, dip, brush or spray techniques. After their application, the resinous tripolymer compositions may be dried, cured or treated according to usual and conventional techniques to remove or counterA effect solvents, vehicles or other` adrnixed ingredients and to effect a solid deposit of the tightly adhering resinous tripolymer layer in the composite structure.

Advantageously, the casting to form composite film structures may be performed suitably by spread coating a dissolved Yor laquer formulation of the functionally basic resinous tripolymer overtlie sulfonated film. While different thicknesses 'may also `be beneficahit is desirable for the applied layer of the resinous tripolymer on the sulfonated surface of any polyctbylel'ie article to be at least about 0.02 vr'nil thick "and even more desirable for many applications for it to have a 4thickness between about 0.1 and 24.0 rnils.V As is apparent, `films may, if desired, be coated onboth of their surfaces in accordance with the invention. In a similar manner, all of the coatable surfaces of other articles such as both `of the inner and outer surfaces of containers and the like or either surface or any desired Yportion of a particular surlface may, if desired, be 'provided with the functionally basic resinous tripoly'mer coating.

Since the application on the 'sulfonat'ed Vpolyethylene 'surface of the functionally basic resinous tripolymer containing functionally bfasic ethe'noid monomer polymerized therein'has somewhat the aspects of a 'metathetical reaction, Ygood `adhesion may generally be achieved with any Vmethod of application; The retention of the applied resinous tripolyrner coating by the polyethylene in the practice of theinv'ention is 'due'no't lonly to thephysical influences of so-called Vander Waals yforces-"and the like but also tothe bonding'ioiiic attraction that is involved between the functionally basic coating and the sulfonated surface ofthe polyethylene.

By way of exemplary illustration, a polyethylene film having a thickness .ofabout lmil which was lof the type Vknownl as Dura-Glearpolyethylenefilm-and which is Massachusetts was surface sulfonated by beingirnrnesed in a 3v percent oleum* (which Vcontains. about 3 percent Yby weight .of free sulfur trioxide dissolved in H2804) at a temperature of "about 50V C. -for aboutive minutes.

ance of 2vinyl pyridine polymerized in the tripolymer molecule. The resinous tripolymer had been prepared by a suspension polymerizaton method using a lauroyl peroxide catalyst. It was dissolved in acetone to form about a 15 percent by weight solution which was cast on the sulfonated polyethylene film to provide a uniform coating, after removal of solvent by drying, that had a thickness of about 1.0 mil. The applied coating could not be stripped olf the film in an adhesion test which consisted of pressing a strip of conventional cellophane tape (such as that which is obtainable under the name Scotch adhesive cellophane tape from the Minnesota Mining and Manufacturing Company) on the coated surface and subsequently pulling the tape away. The coated film had good exibility and did not become embrittled at temperatures as low as -20 C. In addition, its effectiveness as a vapor barrier was improved over that which is characteristic of the uncoated polyethylene film. Further, the coated film had excellent resistance to sur face scuing and was clear and transparent. Its degree of haziness was reduced substantially completely in comparison to that which was present in the original polyethylene film.

Analogous results are obtainable when sulfonated films of essentially linear and unbranched polyethylene and when other surface sulfonated polyethylene articles are coated in accordance with the invention in a manner similar to the foregoing.

It is to be fully understood that the present invention is to be construed and interpreted not by the foregoing didactic description and specification but in the light of what is set forth and defined in the appended claims.

What is claimed is:

1. Method for coating a polyethylene article with a resinous tripolymer of vinylidene chloride, acrylonitrile and a functionally basic ethenoid monomer which comprises sulfonating the surface of the polyethylene and subsequently applying a layer of the resinous tripolymer over the sulfonated surface.

2. In the method of claim 1, sulfonating the surface of the polyethylene by subjecting it to a sulfonating re* agent which is selected from the group consisting of concentrated sulfuric acid containing at least about 98 percent by weight of H2SO4, oleum, anhydrous solutions of oleum and free sulfur trioxide.

3. A method in accordance with the method set forth in claim 2 wherein the sulfonating reagent is comprised of oleum that contains from trace amounts to about percent by weight of dissolved, free sulfur trioxide.

4. A method in accordance with the method set forth in claim 2 wherein the polyethylene is subjected to the sulfonating reagent at a temperature between the freezing point of the reagent and about C.

5. A method in accordance with the method set forth in claim 2 wherein the sulfonating reagent is comprised of oleum that contains from trace amounts to about 10 percent by weight of dissolved, free sulfur trioxide and the polyethylene is subjected to the oleum at a temperature of about 50 C. for about tive minutes.

6. A method in accordance with the method set forth in claim 2 and including the step of washing the sulfonated article free from the sulfonating reagent before applying the coating of the resinous tripolymer.

7. The method of claim 1, wherein the polyethylene article is a film having a thickness which is not in excess of about 20 mils.

S. The method of claim 1, wherein the polyethylene article is a film having a thickness which is between about 0.5 and 5 mils.

9. The method of claim 1, wherein the resinous tripolymer contains between about 40 and 90 percent by weight of vinylidene chloride, between about 5 and 60 percent by weight of acrylonitrile and between about 1 and 15 percent by weight of the functionally basic ethenoid monomer polymerized in the tripolymer molecule.

10. The method of claim l wherein the layer of the resinous tripolymer which is applied over the sulfonated surface has a thickness of between about 0.1 and 2.0 mils.

11. Composite structure which comprises a surface sulfonated polyethylene article coated with a tightly adhering layer of a resinous tripolymer containing between about 40 and 90 percent by weight of vinylidene chloride, between about 5 and 60 percent by weight of acrylonitrile, and between about 1 and 15 percent by weight of a functionally basic ethenoid monomer polymerized in the tripolymer molecule.

12. A composite structure in accordance with the composite structure claimed in claim 11 wherein the surface sulfonated polyethylene article is a film having a thickness between about 0.5 and 5 mils.

13. The composite structure of claim 11 wherein the tightly adhering layer of the resinous tripolymer has a thickness between about 0.1 and 2.() mils.

14. The composite structure of claim 11 wherein the functionally basic ethenoid monomer that is polymerized in the tripolymer molecule is a vinyl pyridine.

15. The composite structure of claim l1 wherein the functionally basic ethenoid monomer that is polymerized in the tripolymer molecule is 2vinyl pyridine.

No references cited. 

1. METHOD FOR COATING A POLYETHYLENE ARTICLE WITH RESINOUS TRIPOLYMER OF VINYLIDENE CHLORIDE, ACRYLONITRILE AND A FUNCTIONALLY BASIC ETHENOID MONOMER WHICH COMPRISES SULFONATING THE SURFACE OF THE POLYETHYLENE AND 